this is where I came in

Idioms  

1. This is where I began, my knowledge dates from this point. For example, Do you have anything more to add, because if not, this is where I came in. This idiom, dating from the 1920s, originally alluded to the continuous showing of a motion picture, with customers entering the theater at any stage while the film was running and leaving when it reached the point where they had started.

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